Line Mixing Research Articles

Dicke narrowing, speed dependence and line mixing of self-broadened CO<sub>2</sub> absorption lines in the 30013 ← 00001 band. Measurements and line shape testing

Dicke narrowing, speed dependence and line mixing of self-broadened CO<sub>2</sub> absorption lines in the 30013 ← 00001 band. Measurements and line shape testing

Measurements of pressure broadening of N2 in the anisotropic tensor component of spontaneous Raman spectra

Spontaneous Raman scattering from nitrogen is well-suited for temperature measurements in combustion experiments, especially at high pressure which increases the Raman signal because of the higher density in the measurement volume. In this work we investigate high density effects on the anisotropic tensor component of ro-vibrational spontaneous Raman scattering, which must be understood to obtain accurate thermometry in high pressure gases using high-fidelity Raman simulations. We measure the collision broadening for the anisotropic component of spontaneous Raman scattering from room temperature nitrogen over the pressure range 10–70 atm for three gas compositions: pure nitrogen, air, and nitrogen in argon. Line broadening coefficients inferred from these measurements were found to be 14 ± 5% larger than the corresponding broadening coefficients of the isotropic Q branch. Air broadening coefficients were found to be very similar and about 2.5% smaller than nitrogen self-broadening coefficients. Argon broadening coefficients were 25% smaller at rotational quantum number 7 and about 50% smaller at rotational quantum number 21. Additionally, we found that our unmixed line model for the O and S branches gave good fits for all pressures, which indicates that line mixing effects are not significant in the O and S branches over this range of pressures. Using indirect experimental evidence we infer that line mixing effects in the anisotropic component of the Q branch were below the threshold set by our experimental spectral resolution at pressures up to 70 atm at room temperature. Assuming that the anisotropic Q branch lines mix like the isotropic lines results in a small systematic error in the inferred temperature at flame temperatures, with the error increasing slowly with pressure. The bias can be easily removed by modeling the anisotropic spectrum separately from the isotropic spectrum. Line mixing effects should be included in the model of the isotropic part of the Raman spectrum, but can probably be neglected in the anisotropic part of the spontaneous Raman spectrum of N 2 for pressures below 400 atm at flame temperatures.

Exploiting line-mixing effects for laser absorption spectroscopy at extreme combustion pressures

A unique spectroscopic strategy has been developed for laser absorption sensing of carbon monoxide (CO) and carbon dioxide (CO2) at extreme pressures (P > 50 atm) relevant to modern combustion devices. The strategy exploits the band narrowing effects of line mixing, which acutely impact spectrally dense regions, such as bandheads, where line spacing is small. Line mixing is shown to counter collisional line-broadening effects that reduce differential absorption at elevated pressures and often limit the pressure range of laser absorption methods. In this work, the R-branch bandheads of CO and CO2, which are only observed at high temperatures relevant to combustion, are targeted near 2.3 µm and 4.2 µm, respectively. Spectral line-mixing models were developed for each bandhead region to account for the collision-induced population transfer rates between rotational energy states over a wide range of elevated temperatures and pressures. Modified-exponential-gap models using the relaxation matrix formalism were shown to capture the thermodynamic dependence of the population transfer rates and enabled scaling. Differential absorption at the bandheads was observed to increase by up to a factor of ten at high gas densities, due to line-mixing effects, enabling detection with relatively narrow-band tunable semi-conductor lasers. With refined spectroscopic models, laser absorption measurements of temperature, CO, and CO2 were demonstrated over a range of high pressures (up to 104 atm) in a sub-scale rocket combustor operated with kerosene and supercritical methane.

Monolithically Integrated Parametric Mixers With Time-Varying Transmission Lines (TVTLs)

Mixers are frequency conversion devices that are widely used in radio applications. This article presents an in-depth theoretical study on the parametric mixing and amplification performance of the time-varying transmission lines (TVTLs) and utilizes the concept to design two TVTL parametric mixers on a commercially available InGaP HBT process. Unlike traditional passive mixers, these parametric mixers can provide a mild conversion gain and achieve a relatively low-noise figure. For the traveling-wave TVTL, the theory of the parametric mixing with the single-sideband condition is reviewed and compared with the double-sideband mixing case. The noise performance analysis shows that the noise figure of the TVTL can be made arbitrarily low to the quantum noise limit through the input reactive termination. The theory serves as a guideline for the design of MMIC parametric mixers. Measurement results have shown that the MMIC traveling-wave TVTL can achieve a maximum of 2.1-dB conversion gain. The mean noise figure was 2.7 dB within the measured frequency band. For the reflective TVTL parametric mixer, the conversion gain and the pump power consumption can be significantly improved. The noise figure, however, is degraded due to the multiple reflections created in the matching circuit. These conclusions are supported by theoretical analyses, ADS harmonic balance simulations, and experiments in this article.

Spectroscopic uncertainty impacts on OCO-2/3 retrievals of XCO2

Estimates of atmospheric carbon dioxide concentration from satellites now span multiple years and are providing information on key processes in the carbon cycle. Methodology for retrieving CO2 from satellite spectra continues to change as understanding of instrument calibration, other atmospheric constituents, and trace gas spectroscopy improve. In this work, we provide an update to linear error estimates for retrieval of carbon dioxide for the Orbiting Carbon Observatory-2/3 (OCO-2/3) due spectroscopic uncertainties. Linear error analysis is used to propagate uncertainties in laboratory conditions and spectroscopic model assumptions to sensitivity in retrieved XCO2 and surface pressure from OCO-2/3. Error estimates due to modeling assumptions for line mixing, collision-induced absorption, and water vapor exhibit noticeable variability across locations and seasons. On the other hand, error estimates due to perturbations in laboratory conditions are nearly constant in space and time.

Temperature-dependent line mixing in the R-branch of the v3 band of methane

High-temperature line mixing behavior was studied for 15 R-branch line manifolds in the v3 band of methane from 3.14 µm to 3.28 µm. The temperature-dependence of the line mixing parameters for the R(4) to R(18) manifolds perturbed by nitrogen were empirically-determined over a range of temperatures from 300–1000 K using Fourier transform infrared spectroscopy in a heated gas cell. A more extensive line mixing investigation of the rovibrational transitions comprising the R(15) manifold was performed at lower pressures (0.3–2 atm) using laser absorption spectroscopy from 300–1600 K in both the heated cell and a high-enthalpy shock tube. Measured spectra were modeled and fit using both a modified exponential gap model and a perturbation theory-based first-order model to capture the pressure- and temperature-dependence of the line mixing effects. The modified exponential gap model was found to sufficiently capture the line mixing behavior at pressures greater than 20 atm, while perturbation theory enables more accurate characterization of the behavior at the lower pressures. The applicability of both the modified exponential gap model and perturbation theory is also discussed with analyses of the fitting residuals and the parameter uncertainties to provide reference for the use of the two empirical models in simulating the R-branch of the v3 band of 12CH4 over a wide range of thermodynamic conditions.

H2-pressure broadening and frequency shifts of methane in the v2+v3 band measured in the temperature range between 80 and 370 K

We have made the measurements of H2-broadened line shape parameters of CH4 transitions in the v2+v3 band located at 2.2 µm. These measurements support atmospheric opacity calculations of cold giant planets, hot Jupiters, and low mass brown dwarfs, for which H2 is the dominant atmospheric constituent. For this reason, a series of spectra of pure CH4 and CH4H2 mixtures were obtained in the Octad (4100–4600 cm−1) region for a wide temperature range between 80 and 370 K using a Bruker 125HR high-resolution Fourier transform spectrometer (FTS) at Jet Propulsion Laboratory (JPL). Three custom-designed gas absorption cells vacuum-coupled to the FTS were used to obtain the spectra. All spectra were fit simultaneously using a multispectrum non-linear least-squares fitting software, which adopts a speed-dependent Voigt line shape using full line mixing taken into account through a relaxation matrix operation. H2 pressure broadened half-widths, and H2 pressure-induced shift coefficients were determined in 11 J-manifolds covering P(4) – R(6) plus the Q-branch. The temperature dependences of the width and shift coefficients were determined based upon the power law and the linear models. Collisional line mixing coefficients for CH4H2 were retrieved for ten transition pairs. For the same transitions, speed dependence parameters for the CH4H2 transitions were found to be smaller than for CH4-air-broadening. We have compiled the retrieved line parameters in an electronic format and reported as supplemental document. This will facilitate the analyses of planetary and exoplanetary atmospheres using ground-, air- and space-based observations (e.g., Keck I and II, SOFIA, JWST).

Line mixing and broadening of carbon dioxide by argon in the v3 bandhead near 4.2 µm at high temperatures and high pressures

Temperature-dependent line mixing and line broadening parameters were empirically-determined for rovibrational transitions (J = 99–145) in the (0000 → 0001) and (0110 → 0111) bandheads of carbon dioxide near 4.2 µm. Collisional effects by argon on the high rotational energy lines (E″ = 3920–8090 cm−1) in the R-branch were studied over a range of temperatures from 1200–3000 K in a shock tube. Measured absorption spectra comprising the target lines in an argon bath gas at near-atmospheric pressures were fit with Voigt profiles to determine line-broadening coefficients, with temperature dependence accounted by a power law. With line broadening established, line-mixing effects were examined at elevated pressures up to 58 atm and similar temperatures, reflecting conditions in high-pressure combustion environments. A modified exponential gap model for line mixing was developed to capture the pressure and temperature dependence of collisional transfer rates for the bandhead region using the relaxation matrix formalism.

3 µm Water vapor self- and foreign-continuum: New method for determination and new insights into the self-continuum

The H2O self- and foreign- in-band continua in the region 3400–3900 cm−1 were experimentally determined for 296 and 353 K from multispectrum fitting results of line parameters using the Hartmann-Tran line profile (HTP) and Rosenkranz line mixing. The continua were extracted from the baselines which were determined in the microwindow-based multispectrum fits. Continua were then obtained by simultaneous fitting of all baselines from measurements containing continuum information. The self-continuum at 296 K was determined from self-broadened measurements and agrees with that determined from air broadened measurements. The overall shape and strength of the new self-continuum agrees with the CAVIAR results between 3600 and 3800 cm-1 but differences exceed the stated uncertainties at higher and lower wavenumbers. Moreover, the new self-continuum is much smoother, has no gaps and is obtained with a high resolution of 2.4 cm−1. The self-continuum was fitted as sum of modeled bound and quasi-bound dimer spectra. From rotational constants, the bound dimer parallel and perpendicular band shapes of the near prolate symmetric top molecule were calculated and used as kernels to fit fundamental wavenumbers, relative band intensities and partitioning of parallel and perpendicular band type, while the integral of the band intensities of the four fundamentals was fixed to published experimental/theoretical data. A dimerization constant for the bound dimer of KDb = 0.026(2) atm−1 and the quasi-bound dimer of KDq = 0.044(5) atm−1 was derived from the fits.The foreign-continuum has no gaps, a spectral resolution of 6–16 cm−1, and is about 40% smaller than the MT_CKD3.2 continuum model. It has a distinctly different shape showing a pronounced P-Q-R branch structure. The foreign-continuum shape is narrower than the monomer band shape which is also true for the MT_CKD3.2 continuum model. The CAVIAR foreign-continuum is much noisier but on average is in good agreement with the new measurements.

Line shape parameters of PH3 transitions: Theoretical studies of self-broadened widths and line mixing effects.

Line mixing effects have been calculated in various parallel and perpendicular bands of self-broadened PH3 lines and compared with recent experimental data. The theoretical approach is an extension to symmetric tops with high inversion barrier of the formalism previously developed for NH3 [Q. Ma and C. Boulet, J. Chem. Phys. 144, 224303 (2016)]. The model takes into account the non-diagonality of the scattering operator within the line space as well as, in a correct way, the double degeneracy of the j, k levels when k ≠ 0. Transitions between such levels should be considered as doublets whose components may be coupled by the line mixing process. It has been shown that, at low pressure, the inversion of the experimental data will strongly depend on the splitting between the two components of a doublet. When it is significant, one can measure independently both the width of one component and the intra-doublet coupling matrix element. Otherwise, one can only measure the sum of these two elements. Comparisons with measurements show that the present formalism leads to accurate predictions of the experimental line shapes.

Infrared Line Collisional Parameters of PH<sub>3</sub> in Hydrogen: Measurements with Second-order Approximation of Perturbation Theory

Measurement of room temperature absorption by PH3–H2 mixtures in the v2 and v4 bands of phosphine (PH3) have been made for low pressures. Fits of these spectra are made for the determination of the width for isolated lines, and line mixing in first-order Rosenkranz approximation. From the previous determinations, we deduce some remarks on the lack of accuracy for the prediction of the collisional process. With the first-order Rosenkranz approximation, the collisional parameters are considered linear with pressure. In this work, we have considered some spectra recorded for three doublets A1 and A2 lines in the v2 and v4 bands of PH3 diluted with higher H2 pressure. We show that the line shifts are non-linear with perturber pressures, which requires testing the fits of the recorded spectra by profiles developed in the second-order approximation of the perturbation theory. Consequently, the first and second-order mixing coefficients are determined and discussed. Also, through this study, we show that the change of the intensities distribution is provided by the populations exchange between the low energy levels for the two components of doublets A1 and A2 lines and is described through the second-order mixing parameter. Thereby, we show the mixing effect on the line width.

Line-shape parameters and their temperature dependences predicted from molecular dynamics simulations for O2- and air-broadened CO2 lines

Requantized classical molecular dynamics simulations (rCMDS) were performed for CO2 highly diluted in O2 at 200, 250, 296 and 350 K using a site-site intermolecular potential. The simulations were made for 0.5 atm of O2 pressure and for a large range of Doppler widths, covering near-Doppler regime to collisional-dominant regime. The Fourier-Laplace transform of the auto-correlation functions of the dipole moment, calculated by rCMDS, leads to the associated spectra of CO2 broadened by O2. Different effects of collisions between CO2 and O2 molecules are included in the simulated spectra. In order to determine the profile parameters of O2-broadened CO2 lines, the rCMDS-calculated spectra were fitted with the speed-dependent Nelkin–Ghatak profile associated with the first-order line mixing. The collisional line broadening coefficient, its speed dependence component, the Dicke narrowing and the first-order line-mixing parameters were retrieved for lines with J up to 50 and for all considered temperatures. The temperature dependences of these line-shape parameters were then deduced using the usual single power law. From results obtained in this work and those obtained for CO2 in N2 [Nguyen et al., J Chem Phys,149, 224301, 2018], the air-broadened line-shape parameters and their temperature dependences for CO2 lines were calculated and compared with literature data showing very good agreement.

Cross-band infrared laser absorption of carbon monoxide for thermometry and species sensing in high-pressure rocket flows

A novel cross-band laser absorption spectroscopy technique has been developed for quantitative measurements of gas temperature and carbon monoxide (CO) in high-pressure, high-temperature rocket combustion flows. The strategy enables a broad range of sensor operability by simultaneously probing rovibrational transitions in both the fundamental and first overtone bands of CO near $$4.98\,\upmu \hbox {m}$$ and $$2.32\,\upmu \hbox {m}$$, respectively, which sustain large differences in temperature dependence despite collisional broadening. Scanned-wavelength modulation spectroscopy methods are integrated for noise rejection in the harsh rocket operating environment. Initial experiments using the cross-band thermometry technique have been conducted on a single-element-injector rocket combustor with RP-2/GOx and $${\hbox {CH}_4}/\hbox {GOx}$$ propellant combinations at pressures up to 75 bar. Measurements of the first overtone bandhead ($$2.32\,\upmu \hbox {m}$$) maintained adequate signal-to-noise at even higher pressures (up to 105 bar), although deviating significantly from spectral simulations. To account for collisional effects at high gas densities, empirical models for line mixing, developed via shock tube studies, were employed to enable quantitative interpretation of measured signals for temperature and CO mole fraction in the rocket combustor.

Shape Change According to the Mixing of Forward and Rear Slope Slice Lines in Graduation Haircuts

Shape Change According to the Mixing of Forward and Rear Slope Slice Lines in Graduation Haircuts

High sensitivity spectroscopy of the O2 band at 1.27 µm: (II) air-broadened line profile parameters

A cavity ring down spectrometer referenced to a frequency comb is used to study the profile of air-broadened O2 lines of the 1.27 µm band. To this aim, spectra of O2 in dry air and in N2 with 2% relative abundance of O2 were measured, in the 7720–7920 cm−1 and 7868–7887 cm-1 spectral ranges, respectively. Spectra were recorded at room temperature and various pressures ranging from 50 to 700 Torr. Detailed line-profile analysis is reported for 85 transitions using the speed-dependent Nelkin-Ghatak model and a multi-spectrum treatment of the two series of spectra. Line mixing was found necessary to be taken into account in the analysis of lines in the region of the Q branch. The derived line parameters including the broadening and shifting coefficients, the speed-dependence components of the collisional line broadening and shifting, as well as the Dicke narrowing parameter are discussed and compared to literature data. In particular, the speed-dependence components of the collisional broadening are found to agree satisfactorily with predictions obtained by molecular dynamic simulations. This obtained set of line-shape parameters should allow for improved modeling of atmospheric spectra in the 1.27 µm spectral region.

Line mixing and broadening in the v(1→3) first overtone bandhead of carbon monoxide at high temperatures and high pressures

Temperature-dependent line mixing and line broadening parameters were empirically-determined for 17 rovibrational transitions in the v(1→3) bandhead of carbon monoxide near 2.3 µm. Collisional effects on the high rotational energy lines (E″ = 5500–8600 cm−1) in the R-branch were studied over a range of temperatures from 1200–3750 K in a shock tube and heated gas cell. Measured spectra comprising the target lines in Ar and CO bath gases were fit with Voigt profiles at near-atmospheric pressures to determine line-broadening coefficients, with temperature dependence accounted by a power law. With line broadening established, line-mixing effects were examined at elevated pressures up to 60 atm at similar temperatures, reflecting conditions in high-pressure combustion environments. A modified exponential gap model for line mixing was developed to capture the pressure and temperature dependence of collisional transfer rates for the bandhead region using the relaxation matrix formalism.

Multiparameter gas sensing with linear hyperspectral absorption tomography

Hyperspectral absorption tomography (HAT) reconstructs the distribution of key gas parameters, including composition, pressure, and temperature, from multi-beam absorbance data with numerous spectral resolution elements. There is a nonlinear relationship between the parameters of interest and the spectral absorption coefficient, which must be incorporated into the tomography algorithm. Nonlinear HAT simultaneously reconstructs the composition and temperature of a gas by minimizing a single nonconvex objective function, which combines the light attenuation and spectroscopy models, using a metaheuristic technique. The time required for this computation depends, strongly, on the assumed heuristics, but the high computational cost limits the problem size and, hence, the obtainable spatial resolution. Conversely, linear HAT reconstructs the absorption coefficient for each measurement wavenumber, individually, exploiting the linear structure of the underlying tomography problem. Local spectra are then post-processed with a spectroscopic model to recover multiple parameters. The linear technique enables accurate reconstructions on a high-resolution grid by way of an established statistical imaging algorithm. Moreover, local spectra can be employed to gauge phenomena such as multi-species broadening and line mixing with a calibrated regression model. We simulate linear and nonlinear HAT and reconstruct experimental absorbance data using the former approach to demonstrate its superior performance. Nonlinear reconstructions required a 100-fold computational effort compared to linear HAT. In our experimental test, we reconstructed the mole fraction, pressure, and temperature of water vapor in a stagnation flow, which represents the first three-parameter laser absorption tomography experiment. Simulated and experimental results in our paper make a comprehensive case for linear HAT compared to the nonlinear method.

Using Data‐Regrouping Methods to Attenuate Shot‐to‐Shot Coherent Interference Noise in Marine Seismic Data

AbstractsUnwanted interference noise often causes problems during seismic reflection data acquisition and processing. In this paper we present two novel data‐regrouping methods that enable improved shot‐to‐shot coherent interference noise attenuation in marine seismic data. The two approaches are based on line mixing and/or shot skipping: (1) for a 3‐D marine seismic reflection data acquisition which is typically carried out by shooting closely spaced parallel lines, we propose to mix the shot gathers from two adjacent sail lines to break up shot‐to‐shot coherent noise. This approach takes advantage of the fact that adjacent sail lines are usually acquired with a significant time interval, resulting in different types of noise; (2) for a 2‐D/3‐D acquisition, we propose an algorithm to skip and reorder the original shots to decrease the continuity of the shot‐to‐shot coherent noise. This approach could also supply more aperture data for the first or last shot from both sides, which is beneficial for the coherent noise removal at the edge area. After regrouping the original data, the coherent noise can then be further randomized or isolated by sorting and transforming to other domains, typically the common p‐shot domain where a random noise attenuation tool could be applied to identify and attenuate such noise. Field data examples confirm the effectiveness of the methods.

Measurement and Modeling of Air‐Broadened Methane Absorption in the MERLIN Spectral Region at Low Temperatures

AbstractThe ability to precisely model methane absorption in the R(6) manifold of the 2ν3 band at atmospheric pressure and temperature conditions is a key technical requirement of the German‐French, Methane Remote Sensing Lidar (MERLIN) space mission. To this end, 27 high‐resolution and high‐signal‐to‐noise‐ratio absorption spectra of air‐broadened 12CH4 were recorded using a variable‐temperature frequency‐stabilized cavity ring‐down spectroscopy apparatus. The measurement conditions corresponded to sample temperature, pressure, and methane molar fraction values spanning 220–290 K, 4–110 kPa, and 4–7 μmol/mol, respectively. The measured spectra were fit using the sum of isolated Hartmann‐Tran profiles with the addition of line mixing. For each line within the manifold, all spectroscopic parameters at room temperature were fixed to our previously obtained values (Delahaye et al., 2016, https://doi.org/10.1002/2015JD024524) and only the temperature dependences of the model parameters were adjusted. The results show that the fitted model agrees with the measured methane absorption to better than 0.3% for the entire R(6) manifold spectral region and to within 0.1% at the online position of the MERLIN for all considered pressure and temperature conditions. A first comparison with ground‐based atmospheric measurement was also made showing significant improvement with respect to existing spectroscopic modeling of methane absorption.

High-pressure and high-temperature gas cell for absorption spectroscopy studies at wavelengths up to 8 µm

An optically accessible gas cell has been designed, manufactured, and tested for the purpose of enabling spectroscopic measurements in the mid-wave infrared at both high pressures and high temperatures. The water-cooled inconel cell has been tested to sustain pressures over 200 atm or operating temperatures above 1200 K. Interchangeable sapphire or calcium-fluoride rods, transmissive up to 8 µm in wavelength, are used to span the thermal gradients at the edge of the heating zone, providing for a low-temperature seal on one end and access to a thermally uniform test section on the other. In this paper, we detail the cell design and validation testing, while highlighting cell capabilities with laser absorption measurements of gas spectra in the 2–8 µm domain at high-pressure ( > 100 atm) and high-temperature ( > 1200 K) conditions. The maximum cell pressure and temperature reached simultaneously was 102 atm and 1030 K. Non-ideal spectroscopic phenomena (e.g. line mixing) are revealed at the extreme conditions investigated.